A typical timing chain drive is shown in FIG. 7, which shows schematically a DOHC (dual overhead cam) engine E in which a timing chain CH is driven by a camshaft sprocket S1, and arranged to drive camshaft sprockets S2 and S3. The slack side of the chain, i.e., the side that moves from the crankshaft sprocket S1 toward camshaft sprocket S2, is in sliding engagement with a movable guide Ga, which cooperates with a tensioner T to apply appropriate tension to the chain in order to prevent vibration of the chain. The tension side of the chain, i.e., the side that moves from camshaft sprocket S3 toward the crankshaft sprocket S1, is in sliding engagement with a fixed guide Gb.
The movable guide Ga is pivoted on a pivot pin P attached to the engine block engine, and is biased by the plunger of tensioner T toward the chain. The fixed guide Gb is mounted on the engine block by mounting bolts Q. A typical movable guide of the kind shown in FIG. 7 is described in, and shown in FIG. 11 of, Japanese Laid-Open Patent Publication No. 2003-214504.
In an internal combustion engine timing drive, the torque load on the camshafts varies cyclically, and the torque load variation causes the tension in the tension side of the chain to vary accordingly. Thus, the tension in the chain on the tension side varies cyclically, and the rate of the cyclic variation in tension depends on the speed of rotation of the crankshaft. For example, in a four-cylinder, in-line engine, the tension in the chain goes through four cycles of variation for each full rotation of the camshafts. In the conventional timing drive in which the tension side of the timing chain slides on a fixed guide, the tensile strength of the chain must be sufficient to withstand the peak value of the tension variation in the chain. Because the chain must withstand the peak tension value, the weight of the timing chain is necessarily large, and the requirement for a heavy timing chain leads to a high overall engine size and weight.
An object of this invention is to provide a timing chain drive in which a lighter and more compact chain can be used, and which makes quieter engine operation possible.